Let people specify a message, and a datetime that that message should be sent to them. Or let them specify recurring tasks like on day x of each month/quarter/year etc. Let people specify that the message should be sent again every day or week until the app receives notification that whatever needs to be done has been done. Such notification of completion could be achieved by the person replying to the SMS. Perhaps that way you could earn a couple of cents for each reminder reply, maybe enough to make it a free service at that level.
I, for example, have a load of boring tasks that I have to do once a month, once a quarter, or once a year. Like taxes. A reminder that told me each time something was due, and then pestered me once a day until I'd done it (because I inevitably procrastinate on boring tasks), would be great.